1. Field of the Invention
The present invention relates to a combined thin film magnetic head used in a VTR or a DAT and, more specifically to a structure of a combined thin film magnetic head in which two or more thin film magnetic heads having overlapping track positions can be fabricated on the same substrate by the same manufacturing process.
2. Description of the Background Art
Recently, a DCC (Digital Compact Cassette) system formed of a stationary head type recorder and reproducer has come to be known as an audio recorder and reproducer for digital signals, which has broad compatibility with a compact cassette used for recording analog signals and is capable of recording and reproducing with tone quality as high as that of a compact disk.
The DCC system includes three different magnetic heads, which are (1), a multichannel head for analog reproduction, (2) a multichannel head for digital reproduction, and (3) a multichannel head for digital recording, as disclosed, for example, in Japanese Patent Laying-Open No. 2-232802.
The multichannel head for analog reproduction has 4 channels, the multichannel head for digital reproduction has 18 channels and the multichannel head for digital recording has 18 channels in order to cope with the automatic reversing mechanism.
An inductive thin film magnetic head is used as the multichannel head for digital recording. Magneto-resistive thin film magnetic heads (hereinafter referred to as MR heads) are used as the multichannel head for analog reproduction and the multichannel head for digital reproduction.
Referring to FIGS. 12 and 13 there is illustrated the structure of the MR head, on a substrate 51 formed of, for example, ferrite or alumina ceramics, a lower yoke 52 of highly permeable magnetic thin film such as Ni--Fe, FeAlSi, Co--Zr is formed. On the upper surface of lower yoke 52, interlayer insulating layers 54a and 54b having approximately trapezoidal cross sections are formed. A biasing lead 54 is arranged between interlayer insulating layers 54a and 54b.
A magneto-resistive element 55 is positioned at the top of the interlayer insulating layers 54a and 54b. Sensing current leads 57a and 57b are connected to magneto-resistive element 55.
An interlayer insulating layer 55a, which is a thin film, is formed on the top surface of magneto-resistive element 55 along the continuous surface of interlayer insulating layers 54a and 54b and of lower yoke 52. Upper yokes 58 and 59 of highly permeable magnetic thin films are formed on the top surface of the interlayer insulating layer 55a, providing a gap 53b above the magneto-resistive element 55.
At a front end which is brought into contact with and slides along a magnetic recording medium, upper yoke 58 and lower yoke 52 opposing each other with interlayer insulating layer 55a interposed therebetween as a gap layer, constitute a front gap 53 for picking up magnetic flux of the magnetic recording medium.
The operation of the MR head having the above described structure is as follows. Namely, variation of the magnetic flux of the magnetic recording medium from front gap 53 is guided to magneto-resistive element 55 at gap 53, and the variation of resistance because of magneto-resistive effect at magneto-resistive element 55 derived from the variation of the magnetic flux is taken by means of leads 57a and 57b.
When MR elements are used as the digital reproducing head and the analog reproducing head for the DCC system, the heads have track width and gap length very much different from each other as shown in Table
TABLE 1 ______________________________________ reproducing head recording head digital analog ______________________________________ track number 18 18 4 track pitch 195 .mu.m 195 .mu.m 900 .mu.m track width 185 .mu.m 70 .mu.m 600 .mu.m length 0.5 .mu.m 0.4 .mu.m 1.8 .mu.m recording 1-5 .mu.m 1-5 .mu.m 5-500 .mu.m wave length ______________________________________
Therefore, it was impossible to use one head both as the digital reproducing head and an analog reproducing head.
In view of the foregoing, in one example of the conventional head configuration, a digital recording head 60, a digital reproducing head 62 and an analog reproducing head 64 are separately formed on separate head substrates, respectively, and the three head chips are combined with the track positions of the three head chips aligned, as shown in FIG. 14.
In this example, digital recording head 60, digital reproducing head 62 and analog reproducing head 64 are positioned in this order from the left, and the upper half a of each track and the lower half b of each track constitute head tracks corresponding to movement of the tape in the directions A and B in the figure, respectively.
Digital recording head 60 includes 18 recording head tracks 61. Digital reproducing head 62 includes 18 reproducing head tracks 63, and analog reproducing head 64 includes 4 reproducing head tracks 65.
A second example of the conventional head configuration is disclosed in Japanese Patent Laying-Open No. 3-250411. The content of this article will be described in the following.
Referring to FIG. 15, this example has a 2-chip configuration in which a digital recording head 67 is formed on one head chip 66, and a digital reproducing head 70 and an analog reproducing head 69 are formed on the same substrate of the other head chip 68.
The structure of the combined reproducing head 100 of this 2-chip configuration is formed in the following manner. Referring to FIG. 16, first, a yoke type MR head 100a for analog reproduction is formed on a substrate 71, a passivation layer 78 is formed thicker than the roughness of the yoke type MR head 100a for analog reproduction, and the roughness on the top surface is made flat by cutting or grinding. Thereafter, a yoke type MR head 100b for digital reproduction is further formed on the flattened passivation layer 78, and then a passivation layer 85 is further formed thereon. Thus, two reproducing heads are stacked on the same substrate.
However, the first example of the conventional head configuration has the following problem. As the density of magnetic recording has come to be higher and higher and the wavelength for recording becomes shorter and shorter recently, it becomes very important to minimize the spacing loss between the magnetic tape medium and the head. However, it is very difficult to minimize the spaces between the three heads and the magnetic tape surface in the first example of the prior art.
In addition, it is also very difficult to assemble the three head chips with the mutual track positions aligned with high precision. Since the cost of the head is proportional to the number of chips, the smaller number of chips is preferred in view of cost.
In other words, a head having two chips at most is desired as the head for the DCC system.
The second example of the prior art has two chips, and therefore it solves the above mentioned problem-of the chip number. However, the process for forming reproducing heads by thin films on the substrate takes at least twice as many steps for separately forming the digital reproducing head and the analog reproducing head. In addition, the total thickness of the thin films laminated on the substrate becomes is at least twice as thick. This increases the number of possible defects derived from increased strain of the substrate, and the thin film peeling between layers and entrance of dust. Consequently, production yields in the wafer process and in the assembly process are considerably decreased compared with the heads formed separately on the semiconductor substrates.